Computer assisted tomography (CT), together with three dimensional image reconstruction procedures, demonstrated in 30 healthy men between the ages of 21 and 81 yr, that the volume of cerebrospinal fluid increased in relation to age and that the volume of gray matter was correlated negatively with age, whereas the volume of white matter in the brain was age invariant. Even in healthy subjects, brain atrophy occurs aging. The volumetric analysis technique also demonstrated brain atrophy, above and beyond that noted with respect to healthy aging, in subjects with Alzheimer's disease, as well as dilation of cerebrospinal fluid spaces. The degree of atrophy was related to psychometric scores for dementia and mental competence. On the other hand, volumetric CT analysis demonstrated no differences, as compared with age matched controls, in brain morphometrics for adults with autism and for young adults with Down syndrome (after data were normalized to height). Cerebrospinal fluid concentrations of homovanillic acid, 5-hydroxyindoleacetic acid, norepinephrine and 3-methoxy 4-hydroxyphenylethylne glycol did not differ significantly between patients with Alzheimer's disease and age-matched controls, and were not correlated with age in healthy controls. On the other hand, spinal fluid concentrations of choline increased with age in healthy controls, and were higher in young adults with Down syndrome than in age matched controls.